The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
When drilling a borehole in order to search for oil or gas resources or in order to use the borehole for underground connection purposes, it is considered essential to accurately monitor and steer the direction of a drill bit such that the borehole is created where desired.
Prior Art U.S. Pat. No. 4,812,977 establishes the method and apparatus of using a strapdown Inertial Navigation System (INS) to map a borehole. The measurement of measured depth independently via a wireline encoder unit is used to restrict the growth of time dependent errors, such as accelerometer and gyroscope drifts. “Strapdown” in this case, means that the sensors are physically and rigidly mounted on the body of the probe, and have no physical isolation from the probes angular rotations and linear accelerations.
One clear disadvantage of this “Strapdown Inertial Navigation” apparatus is that the use of the probe is limited to the dynamic range of the sensors. In gyroscopes, this is typically around 200 degrees/second. During certain drilling operations, angular rotations mainly around the tool long axis (roll axis) can exceed these rates, resulting in Gyroscope saturation and a corresponding loss of the calculated navigation attitude. The survey is then either degraded, or rendered completely invalid.
U.S. Pat. No. 6,714,870 provides a solution to this, whereby the roll axis of the navigation instruments (platform) is decoupled from the probe by use of a mechanical drive unit. The mechanical “platform” is counter-rotated to hold the platform fixed in inertial space, and hence sees much reduced angular rates around the tool long axis. This extends the use of the INS technology to applications where high roll rates can be expected, namely during rotation of the drill-string Gyro MWD (Gyro—Measure While Drilling).
One critical requirement of any wellbore directional system is to be able to find the azimuth of the probe, or when in vertical or near vertical position, to find the azimuth of the “High-Side zero position”, which is termed “Gyro Toolface” or “GTF”.
For further discussion, since this invention can apply to a multitude of sensors, it is important to define the term “Sensitive Axes”. Concerning a “Dynamically Tuned Gyro” or “DTG”, there are 2 “sensitive Axes” orthogonal to each other and to the spin axes. Considering a single axis MEMS gyroscope, a single “sensitive axis” only exists. In this case if 2 “Sensitive Axes” are discussed, it would mean 2× single axis MEMS. Gyros can be used, or 1 dual axis MEMS device to produce the two “Sensitive Axes”.
The ability of any gyro sensor system to find Azimuth or GTF is dependent on the sensitivity and repeatability of the sensors, since the local component of earth rate and local accelerations due to gravity provide the necessary reference information from which the tool attitude is computed. Any non-removable Bias on these sensor readings will appear as an Attitude error. Since the gyroscope has a significant “Bias” coming from manufacturing imperfections, certainly in the range of the earth-rate signal trying to be measured, it is required to eliminate this bias to an insignificant amount.
A good practical way to do this is to rotate the gyroscope physically 180 degrees, using fixed opposing positions, and to measure and compare the output rate of the Gyro in the opposing positions. This procedure for bias nulling is termed “bias nulling” or “indexing”. This procedure has been described for a single Gyro in U.S. Pat. No. 5,806,195. It has however severe limitations in a non-vertical strapdown inertial application.
As the inclination of the tool is increased away from vertical, the system becomes sensitive to unmodelled g-dependent gyro bias. This is because the g-dependent bias acts on the gyroscope in the same cyclic fashion as the earth rate when the platform is rotated to remove gyro bias, and cannot be removed using the “indexing” method. This renders the above-listed invention limited for producing an INS system of the required accuracy for Gyro-MWD.
There is a need for providing a Gyroscopic based navigation device and a method for surveying and directing boreholes which can measure Azimuth (or Gyro Toolface) accurately in all inclinations, and which can provide a stable Gyro Toolface during drilling conditions of possibly high angular rotations around the drillstring, and can additionally provide a continuous INS survey between survey points.
The object is solved by a navigation device for surveying and directing boreholes and a corresponding method according to claims 1 and 9, respectively.
To facilitate this, we disclose here a system which within the narrow confines of a wellbore, can operate in 2 modes. The first mode which can bring two orthogonal “sensitive axes” in a plane orthogonal to the g-vector irrespective of any roll or inclination of the tool. This renders the measurement insensitive to the local acceleration due to gravity, and, when rotating the Gyroscope and applying an Adaptive Kalman Filter to the data, a good bias-free attitude (Azimuth & Inclination) can be reliably computed at all inclinations. The second mode brings mechanically 3 “sensitive Gyro axes” into an orthogonal configuration with respect to each other, enabling an INS platform to be formed for an INS continuous survey of the Borehole between the Gyrocompassing reference points during a survey and/or to provide a Gyro Toolface (GTF) under drilling conditions.
To enable the functionality described in the first mode above, the roll axis of the sensor array is gimballed in the roll axis. This invention allows the Gimbal to operate either as a positional mechanism as described, but also in the second mode of operation it decouples the roll motion of the tool from the sensors. This makes it exceedingly useful for G-MWD applications, where high roll rates can be expected.